Videos with extremely low fps are not multiplexed preferably with audio. This is correct, and compliant with the spec, but not preferable.

Multiple sample_entry (This happens if the stream switches any of its properties in the middle of it) is not fully supported. Fields relative to bitrate does not have correct value in this case. This may be fixed in the future.

The output is suboptimal for streaming. This is not actually problem, because it's unavoidable with 1-pass muxing.